Engineering ceramics are non-metallic, inorganic materials, such as oxides, nitrides and carbides of the metals silicon, aluminum, boron and zirconium. Starting materials for such compositions typically require high temperature processing at above 500.degree. C. to achieve desired densification and desired finished product characteristics. These characteristics include great strength, hardness and substantial chemical inertness which are retained to temperatures in excess of 1000.degree. C.
One type of ceramic includes the SiAlON family, which is characterized by inclusion of the elements Si, Al, O and N. One example SiAlON is .beta.'-SiAlON which has a crystal structure like .beta.-Si.sub.3 N.sub.4, but having some of the silicon atoms replaced by aluminum atoms, and for valency balance, nitrogen atoms replaced by oxygen atoms. Because of their light weight and high resistance to temperature and corrosion, silicon nitride and related composites such as SiAlONs have the potential to be substituted for superalloys containing strategic and critical materials.
The SiAlONs are usually formed by first mixing Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, SiO.sub.2, and another metal oxide such as Y.sub.2 O.sub.3 into a powder mixture. The powder mixture is compacted or otherwise formed into a desired shape, and then fired at approximately 1750.degree. C. for a few hours. The function of the added metal oxide is to react with the alumina and silica (which is present on the surface of each silicon nitride particle) to form a liquid phase which dissolves the reactants and precipitates the product. Y.sub.2 O.sub.3 also functions as a sintering aid which when present in sufficient quantity enables pressureless sintering.
The liquid phase (which still contains dissolved nitrides), cools to form an amorphous glass. The result is a homogenous solution of glass having .beta.'-SiAlON grains dispersed throughout. Typically, a Y.sub.2 O.sub.3 densified .beta.'-SiAlON contains about 15% by volume of Y-SiAlON glass and 85% by volume .beta.'-SiAlON grains.
SiAlONs can also be formed in a Si.sub.2 N.sub.2 O phase, commonly referred to as O'-SiAlON. Such is typically represented as Si.sub.2-x Al.sub.x O.sub.1+x N.sub.2-x. .beta.'-SiAlON is commonly described with the general formula Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z, where solid solution exists for 0.ltoreq.z.ltoreq.4.2. O'-SiAlON offers improved resistance to oxidation at elevated temperatures, while .beta.'-SiAlON possesses mechanical strength similar to .beta. silicon nitride.